The long-term objective of this research program is to understand the relationship between language modality and the organization and acquisition of language. Babbling has long been understood as a phenomenon of early speech development; however, Petitto & Marentette report that prelingually deaf children who receive input from American Sign Language (ASL) produce meaningless, rhythmically-organized gestures that may be manual analogues of vocal babbling. The longitudinal study proposed here will permit the first systematic comparisons of the acquisition of sign and speech during the babbling period and the transition into use of true signs or words. Three subject groups are examined: 1) deaf children of deaf parents, 2) deaf children of hearing parents, and 3) hearing children of hearing parents. These subject groups differ systematically in their language-learning environments: the children of deaf parents receive input from ASL, a naturally-evolved sign language. In contrast, the children of hearing parents receive linguistic input only from speech. Deaf children of hearing parents often receive limited linguistic input from any conventional language, whether spoken or signed. Thus, data on the early gestural development of children of hearing parents will provide a baseline estimate of early gestural development in the absence of linguistic input from a sign language. Five children from each of these three groups will be followed longitudinally from the ages of 5 to 15 months; hour-long videotaped samples of each child interacting with a parent will be collected at biweekly intervals. A gestural coding system has been developed and tested: each gesture will be coded for articulatory properties and communicative function. children's vocalizations will also be analyzed. The following hypotheses will be tested: 1) Prior to the acquisition of true signs, children with sign input will produce meaningless sign-like gestures; 2) The developmental time course for the production of such sign-like gestures will closely resemble that of vocal babbling; 3) If linguistic input is necessary for the development of manual babbling, only children with sign input will produce meaningless, sign-like gestures; 4) But, if children only need auditory of visual feedback from their own productions, then the children of hearing parents should also produce frequent manual babbles. The results of this longitudinal study will significantly enhance our understanding of prelinguistic development in signing and speaking children. Because vocal babbling is the production of word-like, but apparently meaningless vocalizations, further insight into manual babbling can be gained by eliciting the judgments of native signers. In three experimental tasks, panels of native signers will be asked to rate the prelinguistic gestures of deaf and hearing children as being sign-like or not. If input from a conventional sign language such as ASL is necessary for the development of manual babbling, then the prelinguistic gestures of children with sign input should be rated as being more sign- like than the prelinguistic gestures of the children of hearing, non- signing parents.